Shell mold casting and method



A ril 29 1958 H. A. LAURENZ ETAL 2,832,112

SHELL MOLD CASTING AND METHOD Filed May 27, 1955 Inventors iwza (f 5:2 :43;

United htates li atelnt Ofiice SHELL MUM) CASTING AND METHUD Herman A. Laurens and Kenneth E. Spray, Saginaw,

Mich assignors to General Motors Corporation, Detroit, Mich, a corporation of Delaware Application May 27, 1955, denial No. 511,476

lll Claims. (Cl. Z2--l34) This invention relates to foundry casting operations and particularly to a shell mold for use in forming large or elongated castings, such as cast crankshafts for internal combustion engines. i

As is now well known, the shell molding process involves the formation and use of thin-walled dispensable molds and cores composed of sand and plastic binder. This process can be used to produce precision castings in a wide variety of metals.

Essentially the shell molding process consists of using thermosetting plastic or resin as a binder for the sand grains to form rigid molds having high gas permeability, good surface smoothness and dimensional stability. The molding material, which is generally a dry mixture of a major proportion of silica or other refractory oxide sand and a minor proportion of a plastic binder, normally is used in powdered form with no water being added. Phenol formaldehyde and melamine formaldehyde resins are typical examples of the type of therrnosetting binders preferably used. It is desirable that the sand employed be free of clay, moisture and organic matter. Silica flour or other finely comminuted facing materials also may be included in the molding mix to provide the mold with an exceptionally smooth surface.

These sand-resin molds are prepared by allowing the dry mixture of sand and resin to come into contact with a hot pattern for a short period of time. A generally uniform layer of the mix adheres to the pattern surfaces due to the melting of the resin which bonds the sand with which it is intimately mixed, thereby accurately reproducing pattern details. The half patterns, gates and runners usually are permanently fixed on metal plates. Metal patterns normally must be used because they are subjected to elevated temperatures. Pattern temperatures between 350 and 509 Fare preferred, but temperatures ranging as low as 250 F. or lower and as high as 700 F. -i

or even higher may be advantageously employed under particular conditions.

The pattern temperature, the characteristics of the resin and sand used, and the length of time the molding material is allowed to remain in contact with the hot pattern surface determine the thickness of the mold. Mold build-up times ranging from a few seconds to approximately one minute appropriate for various applications. After this short time interval, the excess dry sand and resin are removed, and the closely adhering sandresin layer is preferably cured while in contact with the pattern by subjecting the same to heat within the range of approximately 360 F. to 1500 F. The curing or baking time is relatively short, usually from a few seconds to five minutes. This baking operation results in the conversion of the resinous material to a hard, insoluble binder which securely bonds the sand grains together. After curing of the mold, it is stripped from the pattern and is ready for use. The formed molds are, in effect, thin shells which have sufiicient strength and stiffness to make them suitable for many casting operations.

Conventional types of shrink bobs, whether considered 2,832,112 Patented Apr. 29, 1958 as risers, headers or feeders, generally are satisfactory for use in casting large cast iron parts in green sand molds. In some instances, however, it has been very difficult to produce sound castings of extremely elongated shape if only two runners and two shrink bobs were used, regardless of the type of mold material employed. Moreover, it is especially difiicult to obtain a sound casting of this configuration if the part is cast in a shell mold. Under these circumstances, proper-feeding of the casting is not provided if the shell mold contains conventional shrink bob chambers. This inadequate feeding is evidenced by the presence of surface draws or shrinks, which are external depressions in a parallel pattern. These surface draws usually occur in the vicinity of the parting line.

The aforementioned problems appear to be particularly noticeable in the casting of an elongated malleable iron part in a shell mold. When conventional shrink bobs are used the resultant castings frequently have surface defects which are not found in small parts cast in shell molds. The principal defect consists of one or several small finger-like depressions on the casting -surface, usually terminating in a small circular, rather shallow hole. This type of defect is very objectionable in shell molding operations because of the small finish allowance on shell mold precision castings.

A principal object of the present invention, therefore, is to provide a shell mold form which can be effectively used to produce sound castings of large size or extreme length. A further object of this invention is to provide a novel type or" shrink bob, riser, header or feeder in a shell mold, thereby eliminating difiiculties which would otherwise occur in casting elongated parts in such molds. A still further object of this invention is to provide a method of properly distributing cast iron in an elongated shell mold by means of a shrink bob chamber construction which eliminates surface draws in the casting.

These and other objects are attained in accordance with this invention by means of an elongated shell mold having one or more shrink bob or riser chambers provided with a recessed upper wall. This depressed wall portion in the top of each shrink bob is preferably provided with an opening which permits the molten casting metal to flow upwardly from the interior of the shrink bob chamber into the recess or depression formed by its top wall. The above-described shell mold construction has proved to be particularly effective in casting elongated parts, such as internal combustion engine crankshafts, of malleable iron.

Other objects and advantages of the present invention will more fully appear from the following detailed description thereof, reference being made to the accompanying drawing, in which:

Figure 1 is a longitudinal sectional view of a shell mold to be used for casting a crankshaft for an internal combastion engine in accordance with the invention;

Figure 2 is a vertical sectional view of a shrink bob chamber portion of a shell mold of the type shown in Figure l; and

Figure 3 is a vertical sectional view of a modification of the shrink bob chamber construction shown in Figure 2.

Referring more particularly to the drawing, in Figure l is shown an elongated shell mold 10 for use in casting a crankshaft of an eight-cylinder internal combustion engine of the 90 V-type. walled upper or cope half 12 and a thin-walled lower or drag half 14 which are glued, pasted or clamped together, or otherwise suitably held in proper abutting position during metal pouring operations. The assembled shell mold, the walls of which normally have a thickness between inch and /8 inch, is preferably placed in a horizontal position and the molten casting metal poured while the mold is in this position. Metal chills or sand may be This shell mold consists of a thin-' 3 placed on the upper surface of 'the cope half of the mold to provide for the proper rate of cooling and solidification of the casting.

The casting metal is poured into the vertically extending opening in the sprue portion 16 of the mold and is distributed horizontally to the elongated casting cavity 18 by .means of runner passages 20. These runners extend longitudinally and laterally from the base of the sprue behind the casting cavity, as viewed in Figure l, and com municate with vertically extending shrink bob or riser chambers 21 defined by vertical wall portions 22 of the mold. In the embodiments of the invention shown in the drawing, the shrink bob chambers are of generally cylindrical shape and one is located at each end of the casting cavity 18. The runner passages are shown as communicating with the shrink bob chambers adjacent the parting line 23 of the mold near the lower ends of the chambers. In turn, each shrink bob chamber communicates with one end of the casting cavity 18 by means of one of the gate portions 24 of the mold.

When the molten casting metal is poured, it rises in the vertically extending shrink bob chambers 21, and the shrink bobs or risers formed by the molten metal in these chambers each constitutes a hydrostatic or pressure head which exerts downward pressure. In this manner these risers aid in feeding the casting and absorb shrinkage which would otherwise occur in the metal in the casting cavity 18. Thus close dimensional tolerances of the casting are obtained in the as-cast condition. This is particularly desirable when articles such as crankshafts are precision cast by means of smooth-surfaced shell molds.

As best shown in Figure 2, the efiiciency of the shrink bob may be improved by providing the top Wall of the shrink bob chamber 21 with a central part 26 which extends downwardly into this chamber, thereby forming a cup-shaped depression or recess 28 in the portion of the cope forming the upper walls of the shrink bob chamber. It is necessary that the amount of space in the shrink bob chamber occupied by the depression 28 and downwardly extending wall portion 26 of the shrink bob part of the mold be of adequate size, preferably approximately the same size as the annular area 29 which immediately surrounds the wall 26 and which is occupied with the molten metal after completion of the pouring operation.

The effectiveness of a shrink bob or riser may be improved somewhat by merely increasing the size of the shrink bob chamber or by providing a radially enlarged dome at the upper end thereof. In this manner the additional molten metal in the shrink bob chamber increases the pressure at the base of the shrink bob to improve feeding characteristics of the molten casting metal. However, a shrink bob chamber of sufficient size to be of much additional value or having an enlarged dome is generally impractical to mold because of the difficulty experienced in removing this type of mold from a pattern. Moreover, a large shrink pipe, which extends from the top of the bob down approximately /3 the distance of the gate, frequently will be produced with this type of shrink bob. Generally the bottom /3 of the bob will be of sound metal.

The design of the shrink bob chamber 21 shown in Figures 1 and 2 obviates these difiiculties, however. This shrink bob chamber construction eliminates surface draws because the depression 28 and downwardly extending wall portion 26 create a hot spot or hot point which results in improving feeding characteristics. The presence of this hot spot prevents an oxidized layer or skin from forming over the top of the bob during solidification of the casting metal and allows the atmospheric pressure to be more effectively used in feeding the casting. This hot spot is provided in the approximate areas indicated at 30. With this type of shrink bob chamber the shrinkage is concentrated between the bottom of the recessed upand magnesium powders.

4 per Wall portion 26 and a point appreciably above the gate area. The bottom portion of the shrink bob is of sound metal. Moreover, the wall portion 26 of the shrink bob chamber is not difficult to mold, and shell molds of this configuration can be produced without the necessity of any hand work.

In accordance with the preferred embodiment of the invention, as shown in Figures 1 and 2, additional advantages are obtained if the downwardly extending top wall portion 26 of the shrink bob chamber is provided with a small opening or hole 32 in the upwardly concave lower or bottom portion of this wall. This construction is particularly effective in eliminating surface draws. The opening 32 permits the molten casting metal to enter the depression or recess 28 as the bob is filled during the metal pouring operation. The presence of the molten iron in this recess aids in the creation of a particularly effective annular hot spot at the area 30 adjacent the lower end of the opening 32, thereby keeping the bob open to atmospheric pressure. Casting metal which tends to solidify or form a skin adjacent the opening is retained in a partially molten state at the hot spot location and sinks into the shrinkage hole formed beneath it in the bob. Atmospheric pressure, which can be effectively utilized because of the permeability of the mold, aids in this process. In this manner the solidifying casting metal sloughs down into the annular void, which is normally created by shrinkage of the metal, until the metal is completely solidified.

In the case of shell molds for crankshafts, it is preferred that the opening 32 be approximately inch in diameter. However, the size of this opening may vary appreciably, depending on the size and shape of the part being cast and the pouring temperature and type of casting metal. Hence openings having diameters between approximately A; inch and A2 inch are typical and can be employed in many shell mold casting operations.

As hereinbefore indicated, the riser or shrink bob chamber construction shown in Figures 1 and 2 is particularly effective in eliminating the tendency to form surface draws on elongated shell mold castings, such as crankshafts, formed of malleable iron. As a result, substantial savings can be efiected because of the substantial decrease in the number of castings scrapped.

It has been found that the cope half 12 of a shell mold having the above-described type of shrink bob chamber can be easily molded by the use of a pattern having appropriate recesses to form the depressions 28 and the downwardly extending upper wall portions 26. A sharply pointed pin may be provided at the bottom of each such recess in the pattern so as to extend outwardly therefrom to form the opening 32 in each wall portion 26. The thin shell mold wall or projection which is normally formed around this pin can be easily broken olf immediately after stripping this shell from the pattern. It will be understood, of course, that the cope and drag halves of the shell mold construction described herein may be formed by either blowing or dumping the molding mixture onto the heated pattern.

A modification of the shrink bob chamber construction embodying the invention is shown in Figure 3. This latter construction is similar to that shown in Figure l in that the shrink bob chamber 21 has an upwardly concave wall portion 26 which extends into the chamber to provide a recess or depression 28. However, the wall portion 26 is not provided with the opening 32. Instead the recess 28 is filled with a suitable exothermic material 34, such as a mixture of aluminum and iron powders, iron oxide powder, or mixtures of iron oxide The use of such a material, which gives off heat as the resinous binder in the shell mold burns after the shrink bob chamber has been filled with molten casting metal, helps insure proper feeding of the casting.

The exothermic material is perferably placed in the depression 28 after the shell mold is in position to receive the molten casting metal. However, such exothermic powders are relatively costly, and in most instances it is necessary to add them to the mold by hand. Moreover, the use of an exothermic powder in the foregoing manner does not appear to appreciably reduce surface draw conditions as compared with a similar shrink bob construction from which this material is omitted. In either instance, however, all of the shrinkage in the bob is located between the bottom of wall portion 26 and a point substantially above the gate area.

In both modifications of the shrink bob chamber 21 shown in the drawings, the uppermost portions of the top wall of the chamber, which are located radially outwardly of the downwardly extending central wall portion 26, are provided with one or more venting holes 38. These venting holes permit the ready escape of the gaseous components of the resinous shell mold binder which are evolved during and immediately after pouring of the molten metal. The holes 33 and the surrounding upwardly extending wall portions 40 can be formed in the same manner as the openings 32 in the wall portions 26 shown in Figures 1 and 2. That is, appropriate pins may be provided on the shell mold is formed, and the process hereinbefore described may be employed.

Various modifications in the arrangement and details of the specific embodiments described and shown herein will be apparent to those skilled in the art and are contemplated as within the scope of the present invention as defined in the following claims.

We claim:

1. A shell mold having thin walls defining a shrink bob chamber communicating adjacent one end with a casting cavity in said mold, the wall forming the opposite end of said chamber being provided with a portion extending into said chamber, said portion defining a cupshaped recess above said chamber and containing powdered exothermic material.

2. A shell mold having a shrink bob portion formed by thin walls defining an elongated chamber having a gate opening adjacent its lower end, the upper wall of said chamber extending into said chamber and having an opening through said wall to provide communication between said chamber and the atmosphere.

3. A shell mold having a shrink bob portion comprising thin walls defining a generally cylindrical chamber communicating adjacent one end with a gate for said mold, the wall forming the opposite end of said chamber having a portion extending into said chamber and an opening extending through said portion.

4. A shell mold for use in metal casting operations comprising generally horizontally disposed walls forming a casting cavity therebetween and generally vertically disposed walls forming a shrink bob chamber which extends above said casting cavity, said chamber communicating adjacent its lower end with said cavity, the wall defining the upper end of said chamber extending downwardly into said chamber and being provided with an opening extending from the uppermost portion of said chamber to the atmosphere.

5. A shell mold for use in casting large ferrous metal parts, said shell mold comprising a generally horizontally disposed section defining a cavity having the shape of the part to be cast, a sprue section extending upwardly from said horizontal section and communicating with said cavity, and a vertically extending shrink bob section horizontally spaced from said sprue section, said shrink bob section having an upper wall provided at its central portion with an inwardly extending projection, said wall having an opening extending therethrough laterally outwardly of said projection.

6. A shell mold for use in metal casting operations, said shell mold comprising contoured walls of substantially equal thickness defining a casting cavity having the pattern on which the cope general shape of the finished article to be cast and a generally vertically extending shrink bob chamber communicating adjacent its lower end with one end of said cavity, the wall of said mold forming the top wall of said chamber having a portion extending downwardly into said chamber to thereby provide the upper surface of said top wall with a curvature which is concave upwardly, said top wall having a plurality of openings extending from said chamber through said wall, at least one of said openings communicating with the uppermost portion of said charriber tor venting gases formed during metal pouring operations, and at least one of said openings being located in said downwardly extending portion.

7. A mold for use in casting operations comprising an elongated thin-walled drag half formed principally sand and a thermosetting resin binder and an elongated thin-walled cope half of similar composition positioned against said drag half to form therewith an interjacent mold cavity, said cope half being provided with raised portions adjacent ends of said cavity to form a plurality of shrink bob chambers into which molten casting metal poured into said cavity rises, each of said chambers having a top wall provided with a portion extending downwardly into said chamber, said top walls having venting holes laterally outwardly of said portions to permit the escape of gases evolved from said thermosetting resin binder due to contact between the mold and the molten casting metal, the portions of said top walls which extend into said chambers each having an opening extending therethrough.

8. A shell mold for use in casting a ferrous metal crankshaft for an internal combustion engine, said mold comprising thin-walled cope and drag halves each formed from a mixture of sand and a thermosetting resin which has been heated to cause the resin to bind the sand particles together, said drag and cope halves being maintained in abutting position to provide an elongated castingdefining cavity therebetween, said cope half having a generally vertically extending wall portion shaped to form a sprue opening through which molten casting metal can be poured into said cavity, said cope half having a second generally vertically extending wall portion shaped to form a shrink bob chamber adjacent to and communicating directly with an end of said cavity, said shell mold being provided with a runner passage connecting said sprue opening with said shrink bob chamber adjacent the lower end thereof for feeding the molten casting metal to said cavity, said chamber being provided with a top wall having a central part extending downwardly into said chamber, said top Wall being provided near the uppermost portion of said chamber and outwardly of said central part with a venting hole for permitting the escape of gases generated by contact between the mold and the molten casting metal, said central part of the upper wall having an opening extending therethrough for permitting said molten casting metal to pass through said opening and to rise above said central part.

9. A shell mold for use in metal casting operations, said shell mold comprising a generally horizontally disposed portion defining a cavity having the shape of the part to be cast, a sprue portion provided with a passage communicating with said cavity, a shrink bob portion horizontally disposed from said sprue portion and defining a vertically extending chamber communicating with said cavity and said passage, the upper wall of said shrink bob portion extending downwardly into said chamber to provide a cup-shaped recess at the top of said shrink bob portion, and exothermic material located in said recess.

10. A method of forming a shell mold for use in casting a crankshaft of an internal combustion engine, said method comprising placing a mixture consisting principally of sand and thermosetting resin binder into contact with heated metal patterns to form thin-walled cope and drag halves of a mold shaped to jointly provide a casting cavity having the general shape of a crankshaft,

the pattern forming said cope half having an outwardly extending pattern portionfor forming a shrink bob chamher in said cope half, said outwardly extending portion of the pattern-being provided with a recess and an integral metal pin projecting from said portion defining said recess to extend through the wall of said cope half and provide an opening in the Wall thereof in contact with said portion of the pattern, subsequently curing said drag and cope halves by heating While in contact with said patterns, stripping said halves from said patterns, and thereafter placing said drag half in abutment with said cope half so as to provide a casting-defining cavity therebetween.

References Cited in the file of this patent UNITED STATES PATENTS Mackett Sept. 8, 1942 Galvin et al Nov. 23, 1943 Hites Nov. 23, 1943 Miner Oct. 2, 1951 FOREIGN PATENTS France May 20, 1953 

